I have a darkroom that is about 7' x 14' and use trays up to about 15" x 24" (for platinum prints with 8 x 20 film. My sink is deep enough (about 27" front to back) for these large trays. I have a ledge on the back of my sink which gets a lot of stuff on it. If you get a stainless steel and/or plastic sink, it may not have this. If you build your own, you can build it in.
I recommend that you give yourself more than one sink faucet (I have three). You could connect a simple water filter system from Home Depot to one faucet for cleaner water.
Consider a shallow shelf on the wall above the sink - that's where I have two timers plus developing supplies. String a wire above the sink and below the shelf for hanging prints to dry if you don't have drying shelf space.
One thing I would recommend that you consider is making the sink shallow enough (about 5 or 6" or so) but mount it high enough to sit at rather than stand at. Most darkrooms are designed so you stand at the sink and put "fatigue mats" on the floor. However, I made mine the proper height (both sink and dry area) to sit at with a tall stool on casters. When you spend hours in your darkroom, your back will quickly appreciate the ability to sit down to work. Also, my sink is hand built out of plywood and the front surface is the thickness of two sheets of 3/4" plywood. I bought a length of foam rubber piping insulation at Home Depot that is about 3" around and slit on one side. Fitting the slit of the insulation over the front edge of the sink gives you a great elbow resting surface.
Here's another tip that I found useful - get a little bottle of glow in the dark paint at the drug store and put little dabs of it on key things so you can easily find them in the dark when you are developing film - such as the faucet handles, light switches, and such.
Hope this helps.